The present invention relates to a method for producing a powdered metal gear, and more particularly, to a method for producing a fully dense powdered metal helical gear.
The production of powdered metal articles, including gears, is well-known in the art. One type of powdered metal is selected or different types can be blended together. The powder is disposed in a mold cavity which may be a simple cylindrical preform or may have the profile of the finished product. Next, pressure is applied to create the preform. The preform can then be removed and sintered to produce the part. Where a cylindrical preform is used the preform is placed in another mold and more pressure is applied to form an article having the desired shape. This new preform can then be sintered.
Apparatus for forming helical gears are also known in the art wherein portions of the mold rotate when the preform is impacted to cause the preform to take the shape of the helical gear. For example, such an apparatus having rotating parts for producing powdered metal helical gears is disclosed in U.S. Pat. No. 3,891,367 to Signora. In Signora, the preform has the shape of the actual helical gear to be produced, in contrast to first forming a cylindrical preform which is later transformed into a helical gear.
Goodwin, in U.S. Pat. No. 4,712,411, discloses an apparatus for making a fully dense powdered metal helical gear. Goodwin generally describes producing the helical gear by first creating a cylindrical preform by sintering. The cylindrical preform is then placed in a forming mold wherein the mold cavity has the specific geometry of the helical gear. The preform is then heated and placed in the forming mold where it is axially impacted to both impact the helical toothed shape and also to densify the gear. A disadvantage of the method employed by Goodwin can be that when the preform is impacted a lot of flashing can result as the preform is forced into the shape of the helical gear. Consequently, additional finishing processes can be required to clean up the gear before it is acceptable to a customer.
Both Signora and Goodwin utilize mechanically created pressure to form the gear. However, it is also known to utilize isostatic pressure to form a helical powdered metal gear. For example, Lisowsky, U.S. Pat. No. 5,390,414, discloses a method of manufacturing a helical gear from powered metal using hot and cold isostatic pressure. Like Goodwin, Lisowsky employs a first mold to create a simple cylindrical preform having only the general geometry of the intended gear. A second mold is provided having the specific geometry of the gear and is slightly larger than the preform. The preform is placed inside the second mold, wherein additional powdered metal is provided around the preform to produce a second preform having a helical gear shape. Cold isostatic pressure is used to create both the simple preform and the helical gear preform. After the helical gear preform is made, hot isostatic pressure and/or sintering is employed to create the densified helical gear.
Isostatic pressure forming can generally involve placing a gear preform within a mold cavity having the specific geometry of the helical gear. A rubber bladder is inserted through a center bore in the gear. Fluid is pumped into the rubber bladder at extremely high pressures thus radially expanding the preform against the walls of the mold cavity and causing it to take on the helical gear shape. A disadvantage with isostatic forming is that it can take much longer for the process to fully densify the gear. In hot forming, enormous amounts of pressure can be generated in an instant by impacting the gear axially. In contrast, with isostatic pressure it can take time to build up the pressure and it may be preferable to keep the gear subjected to the pressure for a relatively long time to ensure that the preform fully takes on the specific geometry of the helical gear. Also, for example, obtaining accurate dimensions in the axial direction can be difficult when using isostatic pressure forming. There is generally no mold abutting the axial ends of the gear because the bladder must be inserted through a center bore in the gear. Thus, the axial dimension can be difficult to accurately control. Consequently, more finishing steps can be required to obtain final dimensions having the desired accuracy. Moreover, besides controlling the length of the gear, the lack of control over the axial dimension can also make it more difficult to fully densify the gear. This is because without control over the axial dimension, the gear can experience some undesirable axial expansion in addition to the radial expansion. Consequently, instead of compacting all of the molecules of the gear together, as would occur if both the radial and axial dimensions were controlled, the gear lengthens somewhat which results in a longer and less dense gear.
Accordingly, there is a need for a method of producing fully dense powdered metal helical gears which can eliminate the step of creating a simple cylindrical preform and which can control both the axial and radial dimensions of the gear to create a helical gear with greater density, more accurate axial dimensions and less flashing. Consequently, less finishing steps can be necessary to obtain a superior final product.